This invention relates to a process for the production of urethane elastomers possessing excellent physical characteristics in a one-shot operation.
A known conventional method for producing urethane elastomers comprises a first step of preparing a liquid prepolymer with terminal isocyanate groups by reacting a long-chain diol having a molecular weight of 400 - 3000 with an excess amount of an aromatic diisocyanate and a second step of producing shaped urethane elastomers by mixing the liquid prepolymer prepared in the first step with a curing agent such as a low molecular diol or an aromatic diamine and charging a mold with the mixture under atmospheric or superatmospheric pressure to effect a main chain extension reaction for the prepolymer by reacting the prepolymer with the diol or aromatic diamine. This known conventional method wherein such liquid prepolymer is used has technical merit in that the reaction in the individual steps can easily be controlled to secure the production of urethane elastomers having a desired molecular structure. However, this prior art method encounters many problems in the actual molding operations. According to this prior art method, the prepolymer having terminal isocyanate groups readily absorbs moisture in the air which reacts with the isocyanate groups to form urea as shown by the following reaction formulas: EQU -2 NCO + H.sub.2 O .fwdarw. --NHCONH --+ CO.sub.2
or decomposes the isocyanate group by hydrolysis to form an amine as shown by the following general formulas: EQU --NCO + H.sub.2 O .fwdarw. --NH.sub.2 + CO.sub.2
thus, the prior art method permits dissipation of the isocyanate groups and increase in viscosity of the prepolymer and, in the extreme case, the formation of an insoluble elastomeric film. Such chemical denaturation of the starting material causes considerable deterioration in useful properties of the resulting urethane elastomers. Moreover, the insoluble elastomeric film adheres to vessels and equipment used for the molding operation. Thus, much additional work is required for washing out the adhering film, which obviously causes a significant loss of the starting material.
When a large amount of urethane elastomer is produced according to the prior art method by using an automatic continuous mixing-molding apparatus, the viscosity of the prepolymer is apt to fluctuate with the temperature and the liquid prepolymer readily adheres to or is hardened on the surface of pumps, pipes and the like feeding equipment. Thus, the transported amount of the liquid prepolymer varies according to fluctuation in its viscosity, thus resulting in indefinite proportion of the prepolymer to the curing agent and in changes in the amounts of starting materials used. These are serious drawbacks in putting the prior art method into practice.
Recently, aromatic amines have been suspected of containing hygienically harmful substances. The same applies to aromatic diamines utilizable as curing agents for the liquid prepolymer. According to the known conventional method, an aromatic diamine which is solid under normal conditions, must be fused by heating it at a temperature higher than the melting point of the aromatic diamine prior to mixing with the liquid prepolymer. Under such circumstances, workers are at all times exposed to the danger of inhaling dust or vapor of the harmful aromatic diamine. At present, this danger cannot be perfectly prevented.
It is known that a one-shot method wherein an aromatic diamine, a long-chain diol and an aromatic diisocyanate are reacted directly in one step to produce urethane elastomers can be adopted to overcome the above-mentioned drawbacks and problems in the known conventional methods. At present, however, the one-shot method is applicable only to the case wherein urethane foams or urethane products with low hardness and strength are produced by using a low molecular weight diol as curing agent. Accordingly, the one-shot method cannot be adopted to produce high quality urethane products which are excellent in elasticity, hardness and strength by using an aromatic diamine as curing agent.
To impart high hardness and strength to urethane elastomers, an aromatic diamine must be used as curing agent. However, if an aromatic diamine, a long-chain diol and an aromatic diisocyanate which have been used hitherto in the prior art methods are mixed simultaneously, the reaction between the amino groups and the isocyanate groups will proceed preferentially and a resinous polyurea will be precipitated as insoluble material, as the reaction rate between the amino groups of the aromatic diamine and the isocyanate groups of the aromatic diisocyanate is much greater than that between the hydroxyl groups of the long-chain diol and the isocyanate groups of the aromatic diisocyanate. Although the polyurea precipitated as insoluble solid is presumed to have terminal isocyanate groups, it hardly reacts with the long-chain diol having terminal hydroxyl groups. Accordingly, the diol eventually remains unreacted in the product. Such diol-containing product is a clay-like solid with inferior and unstable properties and is hardly suitable as a urethane elastomer.
The rate of reaction between --OH groups and --NCO groups can be increased and the reaction between --NH.sub.2 groups and --NCO groups can be inhibited by the aid of a catalyst such as triethylenediamine or stannous octanoate in an amount of more than 10 times as much as the amount usually used. The molded products thus obtained are improved to some extent in physical properties, but they involve cracks and bubbles and cannot be used for practical purposes. In addition, the pot life (i.e. the period of time wherein a soft mixture containg curing agent changes into a mixture which can not be poured into a mold of this product is too short to secure enough working time.
In order to produce elastomers with excellent physical properties by such one-shot method, it is necessary to find out such a curing agent as having a very low reactivity with the isocyanate group and being capable of imparting excellent properties to the resulting urethane elastomers. In general, however, urethane elastomers obtained by using a curing agent having a low reactivity with the isocyanate group tend to possess poor physical properties. Thus, curing agents suitable for the production of urethane elastomers by the one-shot operation have not yet been found.